1. Field of the Invention
Exemplary aspects of the present invention generally relate to a transfer device that transfers a toner image formed on an image bearing member to a recording medium and an image forming apparatus including the transfer device.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile capabilities, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image bearing member (which may, for example, be a photosensitive drum); an optical writer projects a light beam onto the charged surface of the image bearing member to form an electrostatic latent image on the image bearing member according to the image data; a developing device supplies toner to the electrostatic latent image formed on the image bearing member to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image bearing member onto a recording medium or is indirectly transferred from the image bearing member onto a recording medium via an intermediate transfer member; a cleaning device then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the unfixed toner image to fix the unfixed toner image on the recording medium, thus forming the image on the recording medium.
Known image forming apparatuses using an intermediate transfer method typically employ a belt-type intermediate transfer member (hereinafter simply referred to as intermediate transfer belt) formed into an endless loop that contacts the photosensitive member, thereby forming a primary transfer nip therebetween. In the primary transfer nip, a toner image formed on the photosensitive member is transferred primarily onto the intermediate transfer belt by a transfer device.
A secondary transfer roller serving as a nip forming member contacts the intermediate transfer belt, forming a secondary transfer nip therebetween, so that the toner image on the intermediate transfer belt is secondarily transferred onto a recording medium in a process known as “secondary transfer process”. A secondary transfer opposed roller is disposed inside the loop formed by the intermediate transfer belt, facing the secondary transfer roller with the intermediate transfer belt interposed therebetween. The secondary transfer opposed roller disposed inside the loop of the intermediate transfer belt is grounded; whereas, the secondary transfer roller disposed outside the loop is supplied with a secondary transfer bias.
With this configuration, a secondary transfer electric field that electrostatically transfers the toner image from the secondary transfer opposed roller side to secondary transfer roller side is formed. The secondary transfer electric field causes the toner image on the intermediate transfer belt to move to a recording medium fed to the secondary transfer nip in appropriate timing such that the recording medium is aligned with the toner image formed on the intermediate transfer belt.
In recent years, a variety of recording media sheets such as paper having a luxurious, leather-like texture and Japanese paper known as “Washi” have come on the market. Such recording media sheets have a coarse surface through embossing process to produce that luxurious impression. Hereinafter, the recording medium having a coarse or embossed surface is referred to as a textured sheet. However, toner does not transfer well to such embossed surfaces, in particular the recessed portions of the surface. This improper transfer of the toner appears as dropouts or white spots in the resulting output image.
Various attempts have been made to prevent improper transfer of the toner under such circumstances. For example, a superimposed bias, in which an alternating current (AC) voltage is superimposed on a direct current (DC) voltage, is supplied as a secondary transfer bias in a superimposed-bias transfer mode to enhance transferability. Depending on the type of the recording medium, the transfer mode can be switched between a DC transfer mode in which a voltage with a DC component is applied to a transfer member and the superimposed-bias transfer mode, thereby transferring sufficiently the toner onto a recording medium regardless of the surface conditions of the recording medium. In the superimposed-bias transfer mode, the superimposed bias, in which a voltage having a predetermined AC component is superimposed on a voltage having a predetermined DC component, is applied to a metal core of a transfer roller serving as a transfer member. Accordingly, good transferability can be achieved for the textured sheet.
Although advantageous and generally effective for its intended purpose, the degree of roughness or the depth of recessed portions of the surface of the textured sheet varies, and thus parameters for image formation, in particular, parameters (hereinafter referred to as sheet dependent conditions) such as a DC component voltage, an AC component voltage, a fixing temperature, and a sheet conveyance speed that depend on the conditions of the sheet also vary. Consequently, in the superimposed-bias transfer mode, it is difficult to set the sheet dependent conditions at a certain level that accommodate all types of sheets. In particular, the voltage having the AC component affects transferability of toner to the recessed portions of the sheet surface, while the voltage having the DC component affects transferability of toner to the projecting portions. If a toner image is transferred based on a set of preset sheet dependent conditions, the preset sheet dependent conditions may be different from desired sheet dependent conditions for a present sheet to be used, and the toner may not be transferred adequately to the recessed portions of the surface of the sheet, causing image defects such as dropouts of toner or blank spots in the resulting image.
Similarly, in an image forming apparatus using a direct transfer method in which the toner image is directly transferred from the photosensitive member to a recording medium, the toner may not be transferred well from the photosensitive member to the recording medium.
In view of the above, there is an unsolved need for an image forming apparatus that is capable of providing good imaging quality regardless of conditions of a recording medium when using a superimposed bias as a transfer bias.